In the production of oil, it is highly desirable to be able to continuously measure the production rate from an individual well, and to measure its total production over defined periods of time. Several problems and limitations of known methods of making these measurements are present. For example, one known problem with rod pump systems is that the traveling valve and standing valve may leak after a short time in service. As a result, a measured build-up of fluid cannot solely be attributed to reservoir inflow, but will also include fluid contributed by the valve leaks. This and other problems contribute to the complexity of the task. Using conventional technology, well flow rate is determined in a number of ways.
A first known method of measuring well flow rate is by use of a dedicated surface meter. Although this method is generally recognized to be the most accurate means for measuring flow rate, the high cost of such dedicated metering equipment has prevented its use on all but a small fraction of the most important producing wells. For custody transfer applications where high accuracy is important, the expense of such dedicated metering may be justified, but in most instances, the high cost of the equipment prohibits their selection and use.
A second known method of measuring well flow rate is by use of a dedicated downhole meter inside the well. This method is often even more expensive than the use of surface mounted equipment because of the high cost of the downhole compatible packaging and of providing connectivity to the downhole meter. This method further suffers from a lack of reliability and difficulty in accessing and servicing downhole equipment.
A third known method of measuring well flow rate is by correlating to pump performance curves for rotating pumps such as progressive cavity pumps or electronic submersible pumps. Load measuring surface equipment is required for measuring the varying loads on the pumping equipment. This method has been shown to often yield inaccurate results as the pump's performance degrades over time and can vary significantly from unit to unit and over time as a result of changes in fluid properties and other dynamic operating conditions. Consequently, this approach is not considered broadly reliable or effective in practice.
A fourth known method of measuring well flow rate is by correlating dimensions and the speed of the pump to develop pump displacement rates with a correction factor. The correction factor is based on several assumptions, each of which is subject to change over time and error for multiple reasons. This method is applicable to rod or beam pumping well applications, and although it does not require additional hardware, it has been shown to give inaccurate results as the pump performance varies due to rod stretch, friction, fluid compressibility, and pump wear over time, which rates vary from well to well. Additional error may be caused by the presence of internal standing or traveling valve leakage and further due to changes in the rate of valve leakage, and which vary from well to well over time.
A related method is disclosed in U.S. Pat. No. 7,212,923, to Gibbs et al. In this disclosure, a well manager is used in combination with a rod-pumping unit to infer liquid production and gas production of a well. The practice of this invention relies on surface load and position measurements of the rod-pumping unit.
A fifth known method of measuring well flow rate is used for managing field production in high well density fields, where wells are in relatively close proximity. In this method, a dedicated surface metering system is put in place to service multiple wells. Surface piping is set up to periodically vent each well's production for a certain time to pass through the flow meter. The periodic routing of the wells can be automated through the use of remote control valves. While the per well expense of such a system is lower than having a dedicated meter for each well, this system suffers the disadvantage in the time required for the flow to fill the feed line and stabilize to allow an accurate reading of the measured well. Another disadvantage is that the readings for any well are only done periodically. Typically, a test is performed in such a system only once every three weeks, and the test will only be allowed to last for a few hours. Another disadvantage of this system is that actual individual well flow rates have been shown to vary substantially over much shorter periods than the period between such tests due to a myriad of factors.
Thus, there remains a need in the management of field production wells for a method of measuring cumulative production over time with reasonable accuracy in the ranges of up to +/−5% or up to +/−10% within each 24 hour period in a cost effective manner. As described, none of the conventional methods consistently satisfy this requirement. The present invention overcomes the several economic and engineering challenges to fill this need, by providing a unique system and method for the measurement of the flow rate of a producing well.